Composite image: The observed quasar (top-left). Light from an accretion disk around a black hole is contaminated by dust clouds (top-right). Adding a polarising filter conceals the clouds, revealing true colour of accretion disk (bottom). (c) M. Kishimoto, M. Schartmann
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Quasars' accretion disks reveal their true colours
Quasars are the brilliant cores of remote galaxies, thought to be powered by supermassive black holes accreting surrounding gas. Central to this picture is a putative accretion disk which is believed to be the source of the majority of the radiative output. A paper in the current issue of Nature by a team of scientists, led by Makoto Kishimoto of the Max Planck Institute for Radioastronomy, verifies a long-standing prediction about the intensely luminous radiation emitted by these accretion disks.
It is well known that the most extensively studied disk model - an optically thick disk which is heated locally by the dissipation of gravitational binding energy - is apparently contradicted by observations in a few major respects. In particular, the model predicts a specific blue spectral shape asymptotically from the visible to the near-infrared, but this is not generally seen in the visible wavelength region where the disk spectrum is observable. A crucial difficulty has been that, towards the infrared, the disk spectrum starts to be hidden under strong, hot dust emission from much larger but hitherto unresolved scales, and thus has essentially been impossible to observe.
The authors report observations of polarised light interior to the dust-emitting region that enable to uncover this near-infrared disk spectrum in several quasars. The revealed spectra show that the near-infrared disk spectrum is indeed as blue as predicted. This indicates that, at least for the outer near-infrared-emitting radii, the standard picture of the locally heated disk is approximately correct.
They believe that further measurements could eventually provide valuable insight into how and where the disk ends.
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